The present invention relates to phantoms for use in the measurement of difficult-to-access structures (especially physiological structures within a living organism) using a high-energy scanning apparatus. The invention also relates to a process and system for measuring these structures.
High-energy imaging methods have long been used for measurement of anatomical structures. Originally, this was done simply by putting a subject between an X-ray source and a photosensitive emulsion.
In recent years, X-ray computed tomography devices have provided a greatly improved imaging capability. Such machines collect X-ray absorption values along various axes in a plane (e.g. by moving the X-ray sensor, and moving the effective position of the X-ray source, around a subject). This set of absorption values provides sufficient data to define a two-dimensional cross-sectional image. By repeatedly advancing the subject and rescanning, multiple two-dimensional images are produced. These two-dimensional images can be combined with the aid of a computer to form a three-dimensional image of the scanned object. (The contour of the three-dimensional image will be somewhat inexact, since the two-dimensional images are separated by "gaps" corresponding to finite increments between successive scans.)
The data obtained from the scanning apparatus is used to construct the two- and three-dimensional images. This technique has been of tremendous utility in diagnostic procedures. However, the scanning machines and methods have not yet achieved their full potential benefits in applications where it is necessary to do precise imaging in the neighborhood of high-density objects. Some of the inherent limitations of these otherwise-useful measurement methods are unacceptable in applications such as precise bone imaging. These errors result from scatter, beam hardening, and partial volume effects.
"Scatter" refers to the unwanted misdirection of some of the X-rays when passing through an object being scanned, causing a somewhat distorted image when some of these misdirected X-rays are sensed by an X-ray sensing device.
"Beam hardening" is a general problem in high-energy imaging. The absorption of different materials varies with wavelength, but the X-ray detectors normally used are not spectrally sensitive. (The X-ray sources normally used for imaging are not monochromatic, and will in fact include an energy spread over a quite significant range of energies.) That is, when bone (or other dense material) is exposed to X-rays, a higher fraction of the lower-energy X-ray photons will be absorbed than of the higher-energy X-ray photons. The reconstruction algorithm may therefore underestimate the density of the region imaged, since the transmitted high-energy photons will mask the fact that a very high percentage of the lower-energy photons have been absorbed or scattered. Thus, failure to correct for beam hardening effects may cause incorrect estimation of material densities. This is particularly a problem when imaging high-density materials, such as bone.
Most computed tomography systems include algorithmic correction for the beam hardening effects of soft tissue. However, the beam hardening effects of bone are not easy to correct for. Conventional methods have not successfully corrected for this, and therefore it has not been possible to get accurate dimensional measurements of bone structures in situ.
"Partial volume effects" result from the fact that the beam will have a finite width or aperture (for example, 40 to 80 millimeters), and therefore the absorption measurements for volumes between the beam source and the image plane will actually be averaged measurements over a certain cross-sectional area.
There are other difficulties in imaging neighborhoods of high-density materials. One difficulty is that the image reconstruction algorithms typically will produce some "smear," due to the fact that the algorithms used to deconvolve the sensed values are imperfect. Thus, for example, an artificially high density may be estimated for the volume within a bone ring.
Another effect is that incorrectly low densities may be estimated for the volume between two bones which are in close proximity to each other (these errors are known as "interosseous lucencies").
A further problem resulting from interosseous lucencies is that estimation of the boundaries between cortical bone (the outermost layer of hard bone) and cancellous bone (the layer of lower density bone which (e.g. in a femur) separates the cortical bone from the marrow) may be very difficult to do correctly. Similarly, estimation of the boundary between cancellous bone and bone marrow may be difficult to do correctly, and estimation of the boundary between cortical bone and surrounding soft tissue may also be very difficult to do correctly. As will be discussed below, precise measurement of these contours (particularly the cancellous/marrow boundary) is extremely useful in certain diagnostic and surgical procedures.
In the past, computed tomography has been used in conjunction with phantoms in order to adjust X-ray computed tomography scanning equipment for variations in attenuation (density) readings. Factors creating a need for this adjustment include aging X-ray tubes and the general sophistication of the equipment used.
Phantoms such as the one described in Zerhouni (U.S. Pat. No. 4,646,334) have been used to calibrate X-ray computed tomography devices using materials of known density. In particular, this patent relates to a phantom containing a material having X-ray attenuation properties similar to a lung nodule. Using this phantom, a computed tomography scanner is calibrated to more selectively detect malignant lung nodules (which have been found to have attenuation properties differing from benign lung nodules). Surrounding the nodule portion of the phantom is other material with shape and attenuation properties similar to the bone and tissue found in and around the lung of a human or non-human animal. This surrounding tissue serves as a means by which the phantom can more closely resemble an animal tissue structure for attenuation calibration purposes. Correction for measurement and sizing errors are not seen to be addressed by this teaching.
Fitting prosthetic hip-stems is an area of application where more accurate measurement capability would be very useful, and where imaging difficulties caused by the proximity of bone are important. The prosthetic hip-stem must be fitted to the inner cortical canal of the femur into which the hip-stem is to be inserted. Accurate measurement is particularly needed it is not desirable to cement the hip-stem to the femur canal.
For example, younger patients, more active patients, and overweight patients will predictably suffer some degradation of artificial hip-stems over time, so that repeated prosthesis operations may be necessary. Similarly, patients who have previously had an infected prothesis will be preferred candidates for non-cemented implants, since reduction of the amount of foreign material lessens the chance of resulting infection.
Thus, it is desirable to provide methods for hip-stem prosthesis which do not require that the hip-stem be cemented, in order to permit removal and replacement of the hip-stems if that should later become necessary. However, non-cemented implants require a much closer fit to the femur canal than would be required for a cemented implant. It is therefore desirable to have an accurate measurement of the femur canal before the surgical procedure is started, to minimize fitting procedures during surgery.
In the past, quantitative computed tomography measurement techniques have been used to generate three-dimensional representative data of the femur canal so that custom hip-stems could be produced. However, the hip-stems manufactured using these techniques did not provide optimal fit, since only a few of the many femur canal curvatures and dimensions were used in the hipstem design process. Specifically, the fit provided was not adequate for non-cemented prosthesis.
In addition, in the prior methods for hip-stem prosthesis, the actual hip-stem design was done by hand. However, U.S. Pat. No. 4,436,684 to White (which is hereby incorporated by reference) suggests a non-invasive method of forming prostheses of skeletal structures internal to a body for use in reconstructive surgery. This method uses data derived from imaging to control a sculpting tool to form an appropriate prosthesis. It would be desirable to be able to use equally rapid methods to form hip-stem prostheses, but the measurement capabilities presently available have not provided sufficient accuracy to permit this.
Thus, it is an object of the present invention to provide a means for correcting errors produced from a high-energy scanning device such as an X-ray computed tomography scanner. Since such errors are significant, a means for eradicating them would be a welcome contribution to the medical/scientific community.